This invention relates to fluids used in treating a subterranean formation. In particular, the invention relates to aqueous energized wellbore treatment fluids, and methods of forming and using such fluids.
Various types of fluids are used in operations related to the development and completion of wells that penetrate subterranean formations, and to the production of gaseous and liquid hydrocarbons from natural reservoirs into such wells. These operations include perforating subterranean formations, fracturing subterranean formations, modifying the permeability of subterranean formations, or controlling the production of sand or water from subterranean formations. The fluids employed in these oilfield operations are known as drilling fluids, completion fluids, work-over fluids, packer fluids, fracturing fluids, stimulation fluids, conformance or permeability control fluids, consolidation fluids, and the like.
Fluid technologies incorporating a gaseous component or a supercritical fluid to form a foam or energized fluid are commonly used in the stimulation of oil and gas wells. For example, some viscoelastic fluids used as fracturing fluids contain a gas such as air, nitrogen or carbon dioxide to provide an energized fluid or foam. Such fluids are commonly formed by injecting an aqueous solution (“base fluid”) concomitantly with a gas, most commonly nitrogen, carbon dioxide or their mixtures, into the formation. Among other benefits, the dispersion of the gas into the base fluid in the form of bubbles or droplets increases the viscosity of such fluid and impacts positively its performance, particularly its ability to effectively induce hydraulic fracturing of the formation, and also its capacity to carry solids (“proppants”) that are placed within the fractures to create pathways through which oil or gas can be further produced. The presence of the gas also enhances the flowback of the base fluid from the interstices of the formation and of the proppant pack into the wellbore, due to the expansion of such gas once the pressure is reduced at the wellhead at the end of the fracturing operation. Other common uses of foams or energized fluids include wellbore cleanout, gravel packing, acid diversion, fluid loss control, and the like.
Foamed and energized fracturing fluids invariably contain “foamers”, most commonly surfactants or blends of surfactants that facilitate the dispersion of the gas into the base fluid in the form of small bubbles or droplets, and confer stability to the dispersion by retarding the coalescence or recombination of such bubbles or droplets. Foamed and energized fracturing fluids are generally described by their foam quality, i.e. the ratio of gas volume to the foam volume. If the foam quality is between 52% and 95%, the fluid is conventionally called foam, and below 52%, an energized fluid. However, as used herein the term “energized fluid” is defined as any stable mixture of gas and liquid, notwithstanding the foam quality value.
The ability to formulate stable energized fluids with rheological properties suitable for fracturing operations becomes increasingly difficult as the temperature of the subterranean formation increases. The matter is worsened when carbon dioxide is present in the gas phase, since carbon dioxide exhibits high solubility in aqueous solutions. The high solubility of carbon dioxide facilitates mass transfer between bubbles and accelerates foam-destabilizing mechanisms such as so-called Ostwald ripening, which ultimately lead to phase separation and to the loss of fluid viscosity. Furthermore, carbon dioxide reacts with water to form carbonic acid. The formation of carbonic acid imposes a low pH environment for the fluid (typically in the range 3.5-4), thus impeding the necessary control of pH for efficient crosslinking with borate ions and often with other metallic ions. Exposure to low pH and high temperatures promotes degradation of the polymeric chains, particularly if polysaccharides are used as viscosifying agents, thus contributing to the referred loss of foam stability and viscosity.
The need to identify suitable chemicals to formulate viscous foams and energized fluids containing carbon dioxide, particularly at elevated temperatures in excess of about 93° C., and more particularly at temperatures in excess of about 121° C., particularly using CO2 or N2, is known to those skilled in the art. Furthermore, there are needs for improved methods to utilize such formulations in the treatment and fracturing of subterranean formations penetrated by a wellbore.
Due to the relatively high cost associated with the foaming agents, there also exists a need to identify efficient surfactants that generate stable foams at reduced cost. A fluid that can achieve the above would be highly desirable. These needs are met at least in part by the following invention.